Lithium ion batteries, since they can realize high energy density, are widely used as power sources of small electronic equipment such as mobile phones and notebook computers, and lately, they have attracted attention also as power sources for large power storage and automobiles.
As a lithium ion battery is increased in size, its energy density tremendously increases. Accordingly, the lithium ion battery is required to have further higher safety. Particularly, in e.g., power sources for large power storage and automobiles, high safety is required. For the reason, safety measures are taken by structural design of a cell and a package, a protective circuit, an electrode material, additives serving for preventing overcharge, and enhancement of a shutdown function of a separator, to ensure safety of a secondary battery.
In a lithium ion battery, an aprotic solvent such as a cyclic carbonate and a linear carbonate is used as the electrolyte solvent. Such an aprotic solvent has a high dielectric constant and a high lithium-ion conductivity; however it tends to have a low flash point.
As one of the means for further improving safety of a lithium ion battery, an attempt has been made to decrease flowability of the electrolytic solution. More specifically, a polymer gel electrolyte, in which an electrolytic solution is held between bridge molecules of crosslinked polymer, and an inorganic gel electrolyte, in which an electrolytic solution is held by an inorganic gelatinizing material such as silica gel, have been investigated.
Polymer gel electrolytes using a crosslinked polymer include those called physical gel and those called chemical gel. In the case of the physical gel, the physical gel is formed by disposing a polymer sheet between a positive electrode and a negative electrode and injecting an electrolytic solution to swell the polymer sheet. In this manner, flowability of the electrolytic solution is suppressed. In the case of the chemical gel, the gel is formed by injecting a crosslinkable polymer precursor and a polymerization initiator together with an electrolytic solution between electrodes and conducting a crosslinking reaction to hold the electrolytic solution between the bridge molecules within the gel. In this manner, flowability of the electrolytic solution is suppressed.
In the meantime, a technique of using a substance, which produces a protective film called an SEI (Solid Electrolyte Interface) on a surface of an electrode, as an additive, is known. This additive is reductively decomposed at a higher potential than a carbonate used as an electrolyte solvent to form an SEI. The SEI has a high lithium ion permeability and a large effect on the charge-discharge efficiency, cycle characteristics and safety of a battery. In addition, SEI can reduce the irreversible capacity of a carbon material and an oxide material of electrodes.
Patent Literature 1 describes production of gel electrolyte battery, in which a gel electrolyte (chemical gel) is produced by polymerizing a polypropylene glycol di(meth)acrylate compound by heating with a peroxide used as a polymerization initiator.
Patent Literature 2 describes production of a lithium polymer battery, in which a solid electrolyte (chemical gel) is formed by crosslinking a crosslinkable material having a ring-opening polymerization functional group through cationic polymerization in a battery container.
Patent Literature 3 describes production of a lithium ion secondary battery, in which an electrolytic solution is gelatinized (to form a chemical gel) by crosslinking a polymer containing a cation polymerizable monomer unit through cationic polymerization in a battery container.
Patent Literature 4 describes production of an electrochemical cell, in which a gel electrolyte is formed by cationic polymerization of a polymer having a cationic polymerizable functional group by charge and discharge, and describes that a SEI is formed at this time.
Patent Literature 5 describes a gelatinous electrolyte battery having a gelatinous electrolyte formed of a matrix polymer (vinylidene fluoride-hexafluoropropylene copolymer), which is swollen with an electrolytic salt-containing nonaqueous electrolyte.
Patent Literature 6 describes a lithium battery using a gelatinous electrolyte formed of an electrolyte, a non-aqueous organic solvent and a hydrophobic inorganic oxide fine particle (silica or titania).